1. Field of the Invention
The field of the invention is fluidized catalytic cracking of heavy hydrocarbon feeds and cyclones for separating fine solids from vapor streams.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. Modern fluid catalytic cracking (FCC) units use zeolite catalysts. Zeolite-containing catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalyst to this low residual carbon level and to burn CO completely to CO2 within the regenerator (to conserve heat and reduce air pollution) many FCC operators add a CO combustion promoter. U.S. Pat. Nos. 4,072,600 and 4,093,535, incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Most FCC's units are all riser cracking units. This is more selective than dense bed cracking. Refiners maximize riser cracking benefits by going to shorter residence times, and higher temperatures. The higher temperatures cause some thermal cracking, which if allowed to continue would eventually convert all the feed to coke and dry gas. Shorter reactor residence times in theory would reduce thermal cracking, but the higher temperatures associated with modern units created the conditions needed to crack thermally the feed. We believed that refiners, in maximizing catalytic conversion of feed and minimizing thermal cracking of feed, resorted to conditions which achieved the desired results in the reactor, but caused other problems which could lead to unplanned shutdowns.
Emergency shutdowns are much like wheels up landings of airplanes, there is no loss of life but the economic losses are substantial. Modern FCC units must run at high throughput, and run for years between shutdowns, to be profitable. Much of the output of the FCC is needed in downstream processing units, and most of a refiners gasoline pool is usually derived directly from the FCC unit. It is important that the unit operate reliably for years, and be able to accommodate a variety of feeds, including very heavy feeds. The unit must operate without exceeding local limits on pollutants or particulates. The catalyst is somewhat expensive, and most units require several hundred tons of catalyst in inventory. Most FCC units circulate tons of catalyst per minute, the large circulation being necessary because the feed rates are large and for every ton of oil cracked roughly 5 tons of catalyst are needed.
These large amounts of catalyst must be removed from cracked products lest the heavy hydrocarbon products be contaminated with catalyst and fines. Even with several stages of cyclone separation some catalyst and catalyst fines invariable remain with the cracked products. These concentrate in the heaviest product fractions, usually in the Syntower (or main FCC fractionator) bottoms, sometimes called the slurry oil because so much catalyst is present. Refiners frequently let this material sit in a tank to allow more of the entrained catalyst to drop out, producing CSO or clarified slurry oil.
The problems are as severe or worse in the regenerator. In addition to the large amounts of catalyst circulation needed to satisfy the demands of the cracking reactor, there is an additional internal catalyst circulation that must be dealt with. In most bubbling bed catalyst regenerators, an amount of catalyst equal to the entire catalyst inventory will pass through the regenerator cyclones every 15 minutes or so. Most units have several hundred tons of catalyst inventory. Any catalyst not recovered using the regenerator cyclones will remain with the regenerator flue gas, unless an electrostatic precipitator, bag house, or some sort of removal stage is added at considerable cost. The amount of fines in most FCC flue gas streams exiting the regenerator is enough to cause severe erosion of turbine blades if a power recovery system is installed to try to recover some of the energy in the regenerator flue gas stream. Generally a set of cyclonic separators (known as a third stage separator) is installed upstream of the turbine to reduce the catalyst loading and protect the turbine blades.
While high efficiency third stage cyclones have increased recovery of conventional FCC catalyst from the flue gas leaving the regenerator they have not always reduced catalyst and fines losses to the extent desired. Some refiners were forced to install electrostatic precipitators or some other particulate removal stage downstream of third stage separators to reduce fines emissions.
Many refiners now use high efficiency third stage cyclones to decrease loss of FCC catalyst fines to acceptable levels and/or protect power recovery turbine blades. However, current and future legislation will probably require another removal stage downstream of the third stage cyclones unless significant improvements in efficiency can be achieved.
We wanted to improve the operation of cyclones, especially their performance on the less than 5 micron particles, which are difficult to remove in conventional cyclones and, to some extent, difficult to remove using electrostatic precipitation.
Based on observations and testing of a transparent, positive pressure cyclone, we realized cyclones had a problem handling this 5 micron and smaller size material. We believed we could improve the performance of those cyclones by drawing underflow in a special way.
Our studies confirmed that FCC cyclones present unique problems, and unique opportunities to improve efficiency. The problems are unique in that FCC cyclones must operate for years, and reliably remove such a spectrum of particulates from flowing gas streams. While catalysts have improved, and do not attrit as much in standardized tests, the FCC environment for catalyst deteriorated. In general, refiners subject the catalyst to more handling, and cause more attrition, by forcing catalyst and vapor to make 4 or 5 turns within a cyclone, rather than 1 or 2. Thus the problem of removing particles in the 5 micron and smaller range has gotten worse, due to increased wear on the catalyst from use of high velocity cyclones to improve efficiency, and from ever stricter limits on particulates in flue gas.
We discovered that the operation of a positive pressure cyclone could be improved by providing a large slot or series of slots on the cyclone wall below the level of the outlet tube for solids underflow. These slots permit circumferential removal of both fines and a limited amount of vapor from the cyclone. This tangential withdrawal may be in addition to, or instead of, the conventional solids withdrawal from the bottom.
In most cyclones, solids are generally withdrawn at right angles to rotational vapor flow within the cyclone, and in the opposite direction to flow of gas from the cyclone outlet.
In our apparatus and process, withdrawing material from an unconventional place (tangential withdrawal) as a supplement to or replacement to conventional underflow produces a cyclone which is unexpectedly effective at removing both large and small particles.